Actuator-integrated force sensor

ABSTRACT

In order to precisely and unambiguously measure braking forces, measurements must be carried out as close as possible to the location where the force is directly introduced onto the brake lining ( 18 ). To this end, the invention provides that the deflection ΔZ of the actuator bottom ( 2 ) is used as a measured variable for the braking force and that the actuator bottom is correspondingly designed for this purpose. The supporting ring ( 3 ) of the actuator ( 1 ) rests in an annular manner on the brake lining ( 18 ). The braking force (F) is centrically applied to the actuator bottom ( 2 ). Various measures are taken in order to eliminate, to the greatest possible extent, the influences of temperature and hysteresis effects as the braking force (F) is converted into a proportional deformation ΔZ.

[0001] The invention relates to a force sensor which is integrated intoan actuator for generating or transmitting a force in the force flux andhas an actuator bottom which is transverse with respect to the forceflux.

[0002] In motor vehicles, the braking function is implemented nowadaysby means of hydraulically activated actuators. In the method designatedas “brake-by-wire”, electrically operated braking devices are used. Insaid braking devices, actuators, i.e. elements in which the brakingforce is generated and by means of which the braking force istransmitted, are activated by means of electromotive step-down drives.The resulting advantages are the individual and variable configurationof the braking process and the possibility of simultaneously performingfurther functions, for example the ABS function (Anti-lock BrakingSystem). As an electromechanical braking system will generally operatewith controlled braking force, the precise measurement of the brakingforce constitutes an essential basis of the performance of the overallsystem. High demands are made of the precision of the system owing tothe synchronous operation. For example, fault tolerances should be <1%even if the braking force is, for example, 5 t. The measuring task isadditionally made considerably more difficult as a result of limitedaccessibility to relevant measurement locations, the small amount offree space in the direction of force and the extremely high spatial andchronological temperature gradients. These aspects rule out the use ofknown force sensors such as, for example, strain gauges.

[0003] The invention is based on the object of enabling precise andunambiguous sensing of braking forces as close as possible to thelocation where the braking force acts. This object is achieved by meansof the combination of features corresponding to claim 1.

[0004] Advantageous refinements can be found in the subclaims. Theinvention is based on the recognition that a force sensor can easily beintegrated into an actuator. The deflection of an actuator bottom or ofa braking piston bottom (a designation by analogy with hydraulicsystems) can be used as a measurement variable for the braking force.The actuator bottom is appropriately configured for this purpose. Theactuator is generally constructed in the form of a hollow cylinder, hasan actuator bottom and also contains a supporting ring with which itbears directly or indirectly on the brake lining of a brake. The brakingforce is generated centrally and applied to the actuator bottom.

[0005] The deformation of the actuator bottom is advantageouslydetermined by means of various measuring methods. A method which issuitable for series production is the capacitive measuring method, theactuator bottom constituting an electrode of a capacitor whosecapacitance which is changed with the deformation is determined. Thecapacitor will therefore generally be a plate capacitor, the electrodewhich lies opposite the actuator bottom being embodied in the form of aplate and being pressed onto a base with spring support so that the hightemperature gradients do not cause any mechanical stresses to betransmitted into the insulator of the electrode and accordingly adefined electrode spacing is ensured, such as is described for examplein the European patent EP 0 849 576 B1.

[0006] As the connecting point between the actuator bottom and the rearpart of the actuator which is generally of cylindrical construction, onthe one hand, and the supporting ring, on the other, is embodied so asto be relatively rigid, the braking force can cause torques to betransmitted to the supporting ring at this point and said torques causethe measurement to be subject to a hysteresis due to friction effects.For this reason, the material cross section at this connecting point isadvantageously reduced by an internal peripheral groove, an externalperipheral groove or by means of a combination thereof so that onlyminimum torques are transmitted.

[0007] The measures which are provided for minimized hysteresis arelikewise suitable for suppressing in the axial direction a temperaturegradient in the actuator bottom due to largely radial introduction ofheat. Axial temperature gradients would cause the actuator bottom tobulge in the direction of the force to be measured and result in anincorrect measurement.

[0008] Exemplary embodiments will be described below with reference toschematic figures which do not restrict the invention.

[0009]FIG. 1 shows an actuator bottom with supporting ring, isothermsand the heat flow being indicated,

[0010]FIG. 2 shows an actuator bottom in the state of rest and in thedeformed state,

[0011]FIG. 3 shows an actuator with grooves provided on the inner andouter peripheries in order to reduce the material cross-section betweenthe actuator bottom and supporting ring,

[0012]FIG. 4 shows a similar view corresponding to FIG. 2, but with ahysteresis-free deformation path of the actuator bottom,

[0013]FIG. 5 shows an actuator with measuring elements for thedeformation Δz,

[0014]FIG. 6 shows an actuator with capacitive measuring device for thedeformation Δz,

[0015]FIG. 7 shows a schematic sectional view of a motor vehicle brakingsystem with a sensor integrated in the frictional engagement and in theactuator.

[0016] A significant element of the invention consists in theintegration of the force sensor in the actuator, the actuator bottombeing used as a measuring element. The elastic deformation of theactuator bottom with corresponding application of a force is thus themeasurement variable at this deformation element. The magnitude of theforce can be inferred from the deformation.

[0017] In order to prevent temperature-induced deformation at theactuator bottom in the measuring direction, i.e. in the direction offorce, it is ensured that at the connecting point between the actuatorbottom and supporting ring the application of the temperature or theintroduction of the heat is such that temperature gradients areminimized in the direction of force, which is equivalent to anapproximately axial profile of the isotherms formed in the direction offorce. The heat flow will thus run inwards almost exclusively in theradial direction.

[0018] In order to prevent hysteresis effects during various loadchanges in which the force is increased and decreased, the generation ortorques and their transmission to the supporting ring are minimized in atargeted fashion. This leads to the connecting point between theactuator and the supporting ring being embodied in a way similar to ajoint. As the actuator bottom serves as a diaphragm-like deformationelement, when force is applied to the connecting point between theactuator bottom and external cylinder or supporting ring a torque willbe generated whose center of rotation is positioned within this Tconnection. This leads to a radial migration of the supporting face ofthe supporting ring on the brake lining. As a result of friction forcespresent at the supporting point, when the load is reduced the originalsupporting position is no longer reached so that hysteresis effectsarise which prevent reproducible measurements. As a result ofappropriately formed grooves which are constructed on the periphery, thetransmission of torques at the point in question is prevented.

[0019]FIG. 1 shows the section through an actuator 1, the actuatorbottom 2 being arranged perpendicularly with respect to the direction offorce. The direction of force is illustrated in FIG. 2. In addition, abase plane 12 as an application point for the force, a supporting ring3, a brake lining 18 and the direction of the heat flow are indicated inFIG. 1. The actuator 1 has an overall cylindrical shape, for the mostpart a hollow cylinder shape. Here, the supporting ring 3 is arranged inthe region of the outer periphery of the actuator bottom 2, in thedirection of force behind the actuator bottom 2. In order to guide theactuator bottom, the hollow-cylinder form is extended opposite thesupporting ring 3, beyond the actuator bottom 2 and counter to thedirection of force. Furthermore, isotherms 11, which characterizevarious temperatures T₁ to T₄, are entered in the actuator bottom 2. Theconstruction corresponding to FIG. 1 does not contain any sensorelements and does not have any features which can prevent temperatureeffects or hysteresis effects. The central feature in FIG. 1 is that theheat flow Q, starting from the brake system with the brake lining 18 onwhich the supporting ring 3 rests, is introduced into the actuatorbottom 2 in such a way that temperature gradients occur in the directionof force in the actuator bottom 2. This leads to temperature-induceddeformations of the actuator bottom, which results in incorrectmeasurement of the force.

[0020]FIG. 2 shows a view corresponding to FIG. 1, the force F, thebraking force, being shown schematically, as is the deformation of theactuator bottom 2 in the form of the deflected actuator bottom 2. Themaximum deflection Δz will occur in the center of the usually radiallysymmetrical component. The deformation which is shown will generate atorque at the connecting point between the actuator bottom 2 and thesupporting ring 3, the center of rotation 10 of said torque beingmarked. As a result of this torque, the surface of the supporting ring 3which rests on the brake lining 18 will be displaced outward when forceis applied. The torques M are indicated schematically.

[0021]FIG. 3 shows a view corresponding to FIG. 1, the heat flow beingintroduced into the actuator bottom 2 virtually perpendicularly to thedirection of force, i.e radially from the outside to the inside, bymeans of an inner peripheral groove 8 and an outer peripheral groove 9.This gives rise to isotherms 11 which are approximately parallel to theforce. As a result of this measure, no temperature-induced deformationsoccur.

[0022]FIG. 4 shows an arrangement corresponding to FIG. 2, in whichmeasures to eliminate torques M occurring when force is applied alsotake the form of peripheral grooves 8 and 9 between the actuator bottom2 and supporting ring 3. In this, the actuator bottom 2 can be deflectedby a maximum absolute value of ΔZ without there being at its outer edgestorques which act on the supporting ring 3 and which cause itssupporting face on the brake lining 8 to migrate outward. The materialcross section is correspondingly reduced by the grooves 8 and 9 so thata construction in the manner of a joint is achieved.

[0023]FIG. 5 shows an actuator arrangement with a measurement of theactuator bottom deflection ΔZ with different sensors. On the one hand,the deflection of the actuator bottom 2 can be measured inductively oroptically with a contactless distance sensor 13. This contactless sensoris, for this purpose, mounted on the base plane 12 which is orientedperpendicularly with respect to the direction of force, and is thusdisplaced by ΔZ in accordance with the central region of the actuatorbottom 2. This displacement is carried out in a contactless way bymoving the sensor close to the actuator bottom 2.

[0024] A further measuring method includes the use of strain sensors 6which are suitable for higher temperatures. These sensors measure, astheir designation suggests, a strain ε, which occurs when a force F actson the actuator bottom 2. Metallic, semiconductor or piezoresistivestrain gauges as well as capacitive strain sensors with silicon surfacemicromechanics can be used as strain sensors. As before, the peripheralgrooves 8 and 9 are illustrated in FIG. 5, together with the bearing ofthe supporting ring 3 on the brake lining 18.

[0025]FIG. 6 shows the actuator 1 with a capacitive measuringarrangement. ΔZ is measured again. The capacitive measuring arrangementcontains an electrode 5 which is positioned on an electrode mount 7. Theelectrode mount is pressed in its outer region onto a base 14 withspring support. The base 14 will remain fixed, even when force isapplied. The spring support is brought about by means of the spring 15which is supported on the rear cover 4. This ensures that the electrode5 is oriented approximately plane-parallel with respect to the actuatorbottom 2 in the position of rest. The actuator bottom 2 thus constitutesthe opposite electrode corresponding to the electrode 5. A change in thedistance between these two electrodes generates a signal which isproportional to ΔZ.

[0026]FIG. 7 shows the entire arrangement of a brake system whichengages on a brake disc 17. The brake linings 18 which are held togetherby the brake caliper 16 are pressed on both sides against the brake disc17 if a spindle 20 exerts a braking force on the actuator 1 byelectromotive means via the motor 19. The electromotive drive is usuallyconnected to a step-down gear mechanism. The spindle 20 transmits thebraking force centrally onto the actuator bottom 2, the motor 19 beingsupported at the rear on a part of the brake caliper 16. In addition,the capacitive sensor 22 is shown schematically. In the illustrationcorresponding to FIG. 7, it is possible to clearly see the heat flowwhich is introduced starting from the contact faces between the brakedisc 17 and brake lining 18 rearward via the brake lining into thesupporting ring 3 and via the latter into the actuator bottom 2. Astemperature differences of several 100° C. can occur here, it becomesclear that temperature-induced deformations can prevent reproduciblemeasurements.

[0027] The following is to be noted with respect to the influence oftemperature and hysteresis. The influence of temperature on a brake canbe enormous as the actuator 1 is heated up considerably in severalseconds during the braking operation. The heat flow Q occurs hereexclusively via the supporting ring 3 and is then distributed into theactuator bottom 2. In the process, considerable axial temperaturegradients occur in the actuator bottom, which is illustrated in FIG. 1.This leads to temperature-dependent bulging ΔZ of the actuator bottom 3,and thus to an incorrect measurement. However, if a turning, in the formof a peripheral groove 8, is made in the interior of the supporting ring3, the heat flow is introduced virtually radially into the actuatorbottom 3, and a temperature-induced axial bulging ΔZ is thus excluded.

[0028] The hysteresis phenomena on the described actuator occur owing tothe relatively rigid connection of the actuator bottom 2 to thesupporting ring 3. The centrally introduced braking force not onlycauses a deflection ΔZ at the actuator bottom 2 but also generates atorque M corresponding to FIG. 2. This torque ensures radial migrationof the supporting face of the supporting ring 3. However, when theloading ceases, a considerable hysteresis effect then occurs owing tothe considerable frictional effects and said hysteresis prevents, to acertain extent, the deformation ΔZ from being reversed in proportion tothe force F. According to the invention, the hysteresis is avoided inthat the rigid connection between the actuator bottom 2 and supportingring is considerably reduced in cross section. Furthermore, theconnection between these two parts is arranged approximately centrallywith respect to the supporting face, as illustrated in FIG. 4. Moreover,a material with a low hysteresis is used to manufacture the sensor.Stainless special steels which can be precipitation-hardened, forexample of the type 17-4PH, are preferably used here. The measurement ofthe deformation ΔZ which is proportional to the braking force isexpediently carried out in relation to the edge of the actuator. Forthis purpose, inductive or optical methods can be used. Capacitivemeasuring principles, as illustrated in FIG. 6, are also particularlysuitable owing to-the high temperatures. The corresponding change incapacitance arises due to a braking-force-dependent change in theelectrode spacing with respect to the actuator bottom 2. A measurementsignal which is proportional to the deformation ΔZ and thus to thebraking force F, results from the radial strain ε of the actuator bottom2. In this case, high-temperature measuring gauges, piezoresistivesensors or capacitive micromechanical strain sensors are possible asstrain sensors.

[0029] The invention is based on the use of the already existingactuator bottom 2 as a deformation element for a direct measurement ofbraking force, and on its geometric configuration in order to measure aforce in a way which is largely independent of temperature and free ofhysteresis.

1. A force sensor with integrated actuator for measuring force in theforce flux with an actuator bottom (2) which is formed transversely withrespect to the force flux and on which the force (F) acts centrally, anda supporting ring (3) arranged peripherally on the outer edge of theactuator bottom in the direction of force for transmitting the force(F), the actuator bottom (2) which can be deformed with the force (F)being part of the force sensor and its detectable deformationconstituting a measure of the force (F).
 2. The force sensor as claimedin claim 1, the deformation being detectable by means of inductive oroptical measuring methods or with metallic, piezoresistive orsemiconductor strain gauges or with strain sensors with silicon surfacemicromechanics.
 3. The force sensor as claimed in claim 1, thedeformation being detectable by means of capacitive measuring methods.4. The force sensor as claimed in claim 3, a central deflection (Δz) ofthe actuator bottom (2) relative to the actuator edge being detectableby means of a plate capacitor arrangement, composed of an electrode (5)positioned on an electrode mount (7) which is pressed, with springsupport, onto a base (14), and a corresponding electrode which isconstituted by the actuator bottom (2).
 5. The force sensor as claimedin one of the preceding claims, the cross section of the connectionbetween the actuator bottom (2) and supporting ring (3) being reduced bymeans of an internal peripheral groove (8) so that minimized temperaturegradients occur in the direction of force in the actuator bottom.
 6. Theforce sensor as claimed in claim 5, a reduction in the material crosssection being provided in order to bring about reduced mechanicalcoupling between the actuator bottom (2) and supporting ring (3). sothat transmission of force from the actuator bottom to the supportingring is minimized.
 7. The force sensor as claimed in one of thepreceding claims, the supporting ring (3) having at least one opening.8. The force sensor as claimed in claim 7, a plurality of openings beingdistributed uniformly over the circumference of the supporting ring (3).